1. Field of the Invention
The present invention relates generally to implantable cardioverter defibrillator systems for treatment of multichamber cardiac dysrhythmias including fibrillation. More particularly, the present invention relates to the independent selection and control of multiple implantable electrode discharge pathways through which multichamber cardioversion or defibrillation countershocks are delivered.
2. Background of the Invention
Cardiac muscle fibrillation is the rapid and asynchronous contraction of individual muscle fibers in the heart. The result is a slightly quivering and non-functional heart muscle. When fibrillation occurs within the lower chambers of the heart or ventricles, blood flow ceases and, if not corrected within minutes, will result in death of the patient. Fibrillation occurring only in the upper chambers of heart or atria results in a diminution of cardiac output that may be symptomatic to the patient. Other forms of cardiac dysrhythmia include ventricular or supraventricular tachycardia which are very rapid organized/synchronous muscle fiber contractions, that impair cardiac output to lesser or greater degrees dependent on cardiac refill times and preload pressures.
Implantable cardioverter and defibrillator systems accomplish the desired treatment of cardiac dysrhythmia by passing a cardioversion or defibrillation countershock through the heart muscle depending on the type of cardiac dysrhythmia diagnosed. An objective of the cardioversion or defibrillation countershock is to immerse as much of the myocardium as possible within the electrical field generated by the countershock. The countershock is a truncated capacitive discharge of electrical energy that generally ranges from 0.1 to 5.0 Joules for cardioversion and from 5 to 40 Joules for defibrillation of the ventricles.
One of the problems in treating cardiac dysrhythmias using cardioversion or defibrillation countershocks is that the strength, and hence the effectiveness, of the electrical field generated across the myocardium may vary greatly. As with any electrical discharge, the electrical field generated by a cardioversion or defibrillation countershock will be a function of the amount and waveform shape of the electrical energy discharged, the location and orientation of the electrodes forming a discharge pathway through which the electrical energy is discharged and the transmicivity or resistivity of the medium through which the electrical field is generated. For electrical cardiac stimulation internal to a human patient, there are a number of viable positions in the patient where implantable discharge electrodes may be positioned.
Various ICD systems employ implantable electrodes positioned external to the heart. These electrodes are placed on the epicardial surface or on the pericardial sac. While these epicardial electrodes are effective in delivering a desired defibrillation countershock, major surgery is required in order to gain access to the epicardium and pericardium for attachment of these electrodes.
Implanted electrodes using vascular access are easier to implant within the heart chambers. These intravenous electrodes are carried on catheters and inserted via venipuncture into the subclavian vein and threaded through the superior vena cava into the right atrium and right ventricle. Intravenous electrodes can be placed at all levels, even passing below the heart into the inferior vena cava.
A third approach uses subcutaneous patches. This method employs tunneling and placement of an appropriate patch electrode within the subcutaneous space requiring minimal surgery. The anatomy of the subcutaneous space provides for placement of a patch electrode virtually anywhere. Effective results for such subcutaneous electrodes have been achieved with placement at the left anterolateral chest wall, epigastrium and left upper quadrant of the abdominal wall.
Various ICD systems contemplate using combinations of electrodes such as those described in U.S. Pat. No. 4,708,145 issued to Tacker, et al. and U.S. Pat. No. 5,107,834 issued to Dahl, et al. These systems employ basic switching mechanisms that generate countershock pulses across a preset combination of electrodes and corresponding discharge pathways, such as firing a pulse across a first set of electrodes and then firing a pulse across a second set of electrodes. In both of these systems, however, the combination of electrodes is predetermined and fixed once the ICD is implanted. The same sequence of switch settings is always used, leading to identical pulses being generated repetitively across the same discharge pathways.
The use of a preset combination of electrodes for cardioversion or defibrillation countershocks also constrains where the electrodes can be placed, even in those systems relying upon multiple electrode placement. For example, in an ICD system with four electrodes: a, b, c and d; if the preset combination of electrodes is a first discharge pathway between electrodes a and b and a second discharge pathway between electrodes c and d, then electrode a must be generally across the myocardium from electrode b, and electrode c must be generally across the myocardium from electrode d. Obviously, the implantation sites for electrodes a, b, c and d must be chosen carefully ahead of time to ensure adequate immersion of the myocardium within the electric field of the countershock.
Another group of ICD systems disclose multiple electrodes with polarity reversing switch mechanisms, such as U.S. Pat. No. 4,800,883 issued to Winstrom, U.S. Pat. No. 4,821,723 issued to Baker, et al., and U.S. Pat. No. 4,998,531 issued to Bocchi, et al. These systems teach monophasic, bi-phasic, multiphasic and even temporally sequential countershock pulses generated across differing sets of electrodes and discharge pathways. Again, however, in each of these systems the differing sets of electrodes used to deliver the countershocks are preset before the ICD is implanted. Regardless of whether the countershock is a mono-, bi-, or multi-phased pulse, the countershock is delivered through the same set of electrodes every time.
Although the use of multiple sets of implanted electrodes located in different positions relative to the heart has provided for several variations in the discharge pathways, and hence the types of electrical fields, that can be generated across the myocardium by a cardioversion or defibrillation countershock, the number and variety of electrical fields are presently limited by the preset combinations of electrodes and discharge pathways available in existing ICD systems. While the number and variety of electrical fields generated by these preset combination of electrodes and discharge pathways are successful in treating many cardiac dysrhythmias, it would be advantageous to provide an ICD with more flexibility in generating a wider variety of discharge pathways and corresponding electrical fields to treat cardiac dysrhythmias.